In high-speed, high-quality electrophotographic printing applications, there are interactions between density (i.e., color gamut) and gloss of the fused images. Moreover, there is a strong influence on the final appearance of the print (i.e., overall gloss, differential gloss) based on the type of media substrate used. In some cases, as shown in U.S. Pat. No. 5,234,783, issued to Ng, the image appearance can also be modified with the addition of a transparent toner overcoat in selected areas.
In a typical printing system, either the transmission density of the toner patches before fusing (if toner patches are laid down in the inter-frame area between paper) is measured from a process control viewpoint, or the reflection density of the fused toner patches on the paper is measured from a process control viewpoint. An in-line gloss measurement device may also be included, to monitor the gloss of fused toner patches on the paper in conjunction with the density measurement. With the gloss and density measurement information and basic document page information (e.g., media substrate type used, color distribution in color management, and digital image content of the printed page), fuser parameters (e.g., fuser nip width and fuser roller temperature) and other electrophotographic element control parameters (e.g., exposure, off-set development voltage, and charger voltage) can be changed to maintain a desirable constant appearance (density, gloss, etc.) of images consistently in print engine usage (within a print run and between print runs), even if different media substrates are used on the same print job.
When a typical process control method is used, one lays down density patches (perhaps including multiple color coverage combination toner patches, rather than only the Dmax (maximum density) color separation patches) on a test sheet. Then, color densities and gloss are measured when the test sheet is diverted to a proof tray, so that the test sheet is out of the mainstream of the printing job flow, which is going to a stacker in a different paper path. Based on measurements of color density and gloss, fuser parameters and electrophotographic element set points may be changed to maintain the desired image appearance.
This conventional practice resolves the issue of off-line linearization that customers once had to perform (although in a non-timely fashion, effecting customer satisfaction because of wasted time) to maintain system quality. However, test sheets (i.e., calibration sheets), that are required to be sent to a proof tray, are considered as waste sheets. Additionally, with conventional methods, further waste sheets are required to have more frequent sampling for more timely control (especially when mixed media and different paper sizes are in the print job stream).
As can be seen, there is a need for improved in-line appearance control that addresses the issue of waste sheets while maintaining the ability for timely appearance control via color density and gloss measurements.